55 research outputs found
Universal decoherence due to gravitational time dilation
The physics of low-energy quantum systems is usually studied without explicit
consideration of the background spacetime. Phenomena inherent to quantum theory
on curved space-time, such as Hawking radiation, are typically assumed to be
only relevant at extreme physical conditions: at high energies and in strong
gravitational fields. Here we consider low-energy quantum mechanics in the
presence of gravitational time dilation and show that the latter leads to
decoherence of quantum superpositions. Time dilation induces a universal
coupling between internal degrees of freedom and the centre-of-mass of a
composite particle. The resulting correlations cause decoherence of the
particle's position, even without any external environment. We also show that
the weak time dilation on Earth is already sufficient to decohere micron scale
objects. Gravity therefore can account for the emergence of classicality and
the effect can in principle be tested in future matter wave experiments.Comment: 6+4 pages, 3 figures. Revised manuscript in Nature Physics (2015
On inference of quantization from gravitationally induced entanglement
Observable signatures of the quantum nature of gravity at low energies have
recently emerged as a promising new research field. One prominent avenue is to
test for gravitationally induced entanglement between two mesoscopic masses
prepared in spatial superposition. Here we analyze such proposals and what one
can infer from them about the quantum nature of gravity, as well as the
electromagnetic analogues of such tests. We show that it is not possible to
draw conclusions about mediators: even within relativistic physics,
entanglement generation can equally be described in terms of mediators or in
terms of non-local processes -- relativity does not dictate a local channel.
Such indirect tests therefore have limited ability to probe the nature of the
process establishing the entanglement as their interpretation is inherently
ambiguous. We also show that cosmological observations already demonstrate some
aspects of quantization that these proposals aim to test. Nevertheless, the
proposed experiments would probe how gravity is sourced by spatial
superpositions of matter, an untested new regime of quantum physics.Comment: 23 pages, 3 figure
General relativistic effects in quantum interference of photons
Quantum mechanics and general relativity have been extensively and
independently confirmed in many experiments. However, the interplay of the two
theories has never been tested: all experiments that measured the influence of
gravity on quantum systems are consistent with non-relativistic, Newtonian
gravity. On the other hand, all tests of general relativity can be described
within the framework of classical physics. Here we discuss a quantum
interference experiment with single photons that can probe quantum mechanics in
curved space-time. We consider a single photon travelling in superposition
along two paths in an interferometer, with each arm experiencing a different
gravitational time dilation. If the difference in the time dilations is
comparable with the photon's coherence time, the visibility of the quantum
interference is predicted to drop, while for shorter time dilations the effect
of gravity will result only in a relative phase shift between the two arms. We
discuss what aspects of the interplay between quantum mechanics and general
relativity are probed in such experiments and analyze the experimental
feasibility.Comment: 16 pages, new appendix, published versio
Probing anharmonicity of a quantum oscillator in an optomechanical cavity
We present a way of measuring with high precision the anharmonicity of a
quantum oscillator coupled to an optical field via radiation pressure. Our
protocol uses a sequence of pulsed interactions to perform a loop in the phase
space of the mechanical oscillator, which is prepared in a thermal state. We
show how the optical field acquires a phase depending on the anharmonicity.
Remarkably, one only needs small initial cooling of the mechanical motion to
probe even small anharmonicities. Finally, by applying tools from quantum
estimation theory, we calculate the ultimate bound on the estimation precision
posed by quantum mechanics and compare it with the precision obtainable with
feasible measurements such as homodyne and heterodyne detection on the cavity
field. In particular we demonstrate that homodyne detection is nearly optimal
in the limit of a large number of photons of the field and we discuss the
estimation precision of small anharmonicities in terms of its signal-to-noise
ratio.Comment: 8 pages, 2 figures, RevTeX
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